Overvoltage Clamp in Regulators

ABSTRACT

A regulator for providing a load current at a regulator output voltage to a load at an output of the regulator is described. The regulator has a differential input stage to provide a differential output voltage based on a reference voltage and based on the regulator output voltage. Furthermore, the regulator has an output driver to generate a control signal based on the differential output voltage. In addition, the regulator has a pass transistor to provide the load current in dependence of the control signal. The regulator also has clamping circuitry to sense an overvoltage indication which indicates that the pass transistor is being turned off. Furthermore, the clamping circuitry clamps the differential output voltage to a clamping voltage, if the overvoltage indication indicates that the pass transistor is being turned off.

TECHNICAL FIELD

The present document relates to regulators, notably to low-dropout (LDO)regulators. In particular, the present document relates to regulatorshaving a fast recovery subject to an overvoltage condition.

BACKGROUND

In LDOs, notably in LDOs employing a relatively high gain multi-stageamplifier as an error amplifier and using Miller compensation,relatively slow recovery from an overvoltage condition may occur due tooperating point disturbance after a load transient or after any otherexcitation causing feedback voltage deviation from normal operation. Ifthe LDO is disturbed prior to full recovery of the internal nodes of theLDO (e.g. by a load current request), relatively high voltage dips maybe observed at its output, because the LDO is out of regulation. Thismay lead to reduced regulator output voltage levels at the output of theLDO and may cause resets in the circuitry which is supplied by the LDO.

Such situations may occur e.g. for pulse train like load currents whichperiodically toggle between a relatively high load current IMAX and arelatively low load current or no load current. In particular, suchsituations may occur if the time interval between a request for arelatively high load current and a request for a relatively low loadcurrent is shorter than the time which is needed for the LDO's internalnodes to recover to their target operation points after an overvoltagecondition.

SUMMARY

The present document addresses the technical problem of providing aregulator circuit which exhibits a fast and stable recovery subject toan overvoltage condition.

According to an aspect, a regulator is described. The regulator is usedfor providing a load current at a regulator output voltage to a load atan output of the regulator. Typically, the regulator is configured toregulate the regulator output voltage in accordance to a pre-determinedreference voltage, which is set at an input of a differential inputstage of the regulator. In particular, the regulator may comprise alow-drop out (LDO) regulator.

The regulator comprises a differential input stage which is configuredto provide a differential output voltage based on a reference voltageand based on the regulator output voltage. In particular, thedifferential input stage may be configured to provide the differentialoutput voltage based on a difference between the reference voltage and afeedback voltage which is derived from the regulator output voltage. Thefeedback voltage may be derived from the regulator output voltage usinga voltage divider. As such, the feedback voltage may be proportional tothe regulator output voltage, wherein the proportionality factor may bedefined by the resistor ratio of the voltage divider.

Furthermore, the regulator comprises an output driver which isconfigured to generate a control signal (e.g. a gate signal) based onthe differential output voltage. In addition, the regulator comprises apass transistor which is configured to provide the load current independence of the control signal. The pass transistor typicallycomprises a gate and the control signal may be applied to the gate ofthe pass transistor. In particular, the pass transistor may be a metaloxide semiconductor (MOS) transistor, notably a p-type MOS transistor.By changing a voltage level of the control signal, the load currentthrough the pass device may be modified, i.e. the pass transistor may beturned on or off. In case of a p-type MOS transistor, increasing thecontrol signal typically leads to a reduction of the load current anddecreasing the control signal typically leads to an increase of the loadcurrent.

The regulator further comprises clamping circuitry which is configuredto sense an overvoltage indication which indicates that the passtransistor is being turned off. The overvoltage indication may be sensedby sensing the control signal which is applied to the pass transistor,thereby providing a fast indication of whether the pass transistor isbeing turned off. If it is determined that the regulator is turning offthe pass transistor (completely), this typically means that therequested load current has dropped significantly so that the loadcurrent charges an output (notably an output capacitor) of theregulator, thereby increasing the regulator output voltage above adesired level (which is indicated by the reference voltage). Hence, thefact that the pass transistor is being turned off (completely) istypically in indication of an overload situation at the output of theregulator.

The clamping circuitry is further configured to clamp the differentialoutput voltage (at the output of the differential input stage) to aclamping voltage, if the overvoltage indication indicates that the passtransistor is being turned off. By clamping the differential outputvoltage to a clamping voltage, an overshoot of the differential outputvoltage and a corresponding overshoot of the control signal may beavoided, thereby avoiding a complete turn-off of the pass transistor. Inparticular, it may be achieved by clamping of the differential outputvoltage that the regulator and in particular the different stages of theregulator are maintained relatively close to their normal operationpoints. As a result of this, a fast and stable recovery subject to anovervoltage condition may be achieved.

Typically, the differential output voltage should be close to zero(thereby indicating that the feedback voltage corresponds to thereference voltage). As such, the clamping voltage may be relativelyclose to zero. By way of example, the clamping voltage may depend on ormay correspond to the voltage drop of a forward-biased (MOS) diode.

The clamping circuitry may be configured to sense the overvoltageindication by determining a mirrored version of the control signal. Bydoing this, the overvoltage indication may be determined in a preciseand timely manner, thereby ensuring a fast reaction of the clampingcircuitry to an overvoltage condition.

The pass transistor may be coupled to an input voltage (also referred toas a supply voltage). The output driver may comprise an auxiliarytransistor (notably a p-type MOS transistor) and a lower drivertransistor (notably an n-type MOS transistor), which are arranged inseries between the input voltage and ground. The control signal may beprovided at a midpoint between the auxiliary transistor and the lowerdriver transistor, wherein this midpoint may be referred to as a controlmidpoint. A voltage which is applied to a gate of the lower drivertransistor may depend on the differential output voltage. As such, theoutput driver may generate the control signal based on the differentialoutput voltage.

The output driver may further comprise an upper driver transistor(notably a p-type MOS transistor), wherein a source of the upper drivertransistor may be coupled to the input voltage and wherein a drain and agate of the upper driver transistor may be coupled to the controlmidpoint. Furthermore, the regulator may comprise a bias current sourcewhich is configured to provide a bias current and a bias transistor(e.g. a p-type MOS transistor) which is arranged in series with the biascurrent source between the input voltage and ground. The auxiliarytransistor and the bias transistor may form a current mirror. Thecombined use of an auxiliary transistor and an upper driver transistormay be beneficial for setting an operation point of the pass transistor.

The clamping circuitry may comprise an upper sensing transistor (notablya p-type MOS transistor) and a lower sensing transistor (notably ann-type MOS transistor) which are arranged in series between the inputvoltage and ground. Gates of the lower driver transistor and the lowersensing transistor may be coupled to one another. Furthermore, gates ofthe auxiliary transistor and the upper sensing transistor may be coupledto one another. As such, a (scaled) version of the output driver may beprovided for sensing the overvoltage indication. By doing this, theovervoltage indication may be provided at a midpoint between the uppersensing transistor and the lower sensing transistor. The overvoltageindication thereby corresponds to a (scaled) version of the drivesignal, thereby providing a precise and immediate indication on whetherthe pass transistor is being turned off.

A size (notably a width-to-length ratio) of the upper sensing transistormay be greater than a size of the auxiliary transistor. Furthermore, asize (notably a width-to-length ratio) of the lower sensing transistormay be equal to the size of the lower driver transistor. By selectingthe upper sensing transistor to be greater than the auxiliarytransistor, it may be ensured that the overvoltage indication is setprior to a full turn-off of the pass transistor, thereby ensuring anearly activation of clamping and thereby enabling a fast recoverysubject to an overvoltage condition.

Alternatively or in addition, the clamping circuitry may comprise acomparator transistor (notably a p-type MOS transistor) and a referencecurrent source, wherein the reference current source is configured toprovide a reference current. The reference current may be tuned todefine the control signal at which the overvoltage indication is set (toindicate that the pass transistor is being turned off). The comparatortransistor and the reference current source are arranged in seriesbetween the control midpoint and ground. A gate of the comparatortransistor is coupled to an offset version of the input voltage. Theoffset version of the input voltage may be generated using one or morediodes, which are arranged in a forward biased manner between the inputvoltage and the gate of the comparator transistor. The overvoltageindication may be provided at a midpoint between the comparatortransistor and the reference current source.

The clamping circuitry may comprise a clamping diode which is set orarranged to couple an output of the differential input stage to ground,if the overvoltage indication indicates that the pass transistor isbeing turned off. The clamping voltage may then depend on or maycorrespond to a diode voltage drop at the clamping diode. As such, itmay be ensured that the differential output voltage stays close to itsnormal operation point (even in case of an overvoltage condition).

The overvoltage indication may take on a low level and a high level. Ahigh level may indicate that the pass transistor is being turned off. Onthe other hand, a low level may indicate that no clamping of thedifferential output voltage should occur (such that the regulation loopof the regulator is not disturbed).

The clamping diode may comprise a clamping transistor (notably an n-typeMOS transistor). A gate of the clamping transistor may be coupled to theoutput of the differential input stage, and a source of the clampingtransistor may be coupled to ground. The clamping circuitry may beconfigured to couple or decouple a drain of the clamping transistor toor from the gate of the clamping transistor in dependence of the levelof the overvoltage indication. By doing this, the clamping to theclamping voltage may be activated or deactivate in an efficient manner.The clamping voltage depends on or corresponds to the gate-sourcevoltage of the clamping transistor in this case.

The clamping circuitry may comprise a first transistor (notably ann-type MOS transistor) and a second transistor (notably an n-type MOStransistor). A drain of the first transistor may be coupled to a node atwhich the overvoltage indication is provided (e.g. to the midpointbetween the upper sensing transistor and the lower sensing transistor,or to the midpoint between the comparator transistor and the referencecurrent source). A drain of the second transistor may be coupled to theoutput of the differential input stage. Furthermore, gates of the firsttransistor and the second transistor may be coupled to one another. Thegate of the first transistor may be coupled to the drain of the firsttransistor, a source of the first transistor may be coupled to ground,and a source of the second transistor may be coupled to the drain of theclamping transistor. As such, clamping may be activated or deactivate inan efficient and precise manner.

The regulator typically comprises an intermediate amplification stagewhich is coupled to an output of the differential input stage and whichis configured to generate an intermediate voltage based on thedifferential output voltage. The output driver may be configured togenerate the control signal based on the intermediate voltage.

According to a further aspect, a method for providing a load current ata regulator output voltage to a load is described. The method comprisesproviding a differential output voltage based on a difference between areference voltage and a feedback voltage, wherein the feedback voltageis derived from the regulator output voltage. Furthermore, the methodcomprises generating a control signal based on the differential outputvoltage, and providing the load current in dependence of the controlsignal using a pass transistor. In addition, the method comprisessensing an overvoltage indication which indicates that the passtransistor is being turned off, and clamping the differential outputvoltage to a clamping voltage, if the overvoltage indication indicatesthat the pass transistor is being turned off.

It should be noted that the methods and systems including its preferredembodiments as outlined in the present document may be used stand-aloneor in combination with the other methods and systems disclosed in thisdocument. In addition, the features outlined in the context of a systemare also applicable to a corresponding method. Furthermore, all aspectsof the methods and systems outlined in the present document may bearbitrarily combined. In particular, the features of the claims may becombined with one another in an arbitrary manner.

In the present document, the term “couple” or “coupled” refers toelements being in electrical communication with each other, whetherdirectly connected e.g., via wires, or in some other manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in an exemplary manner with referenceto the accompanying drawings, wherein

FIG. 1a illustrates an example block diagram of an LDO regulator;

FIG. 1b illustrates an example block diagram of an LDO regulator withMiller compensation;

FIG. 1c shows example measurement signals at an LDO regulator;

FIG. 2 shows a block diagram of an example LDO regulator with clampingcircuitry;

FIG. 3 shows a circuit diagram of an example LDO regulator with clampingcircuitry;

FIG. 4 shows a circuit diagram of an example LDO regulator with modifiedclamping circuitry;

FIG. 5 shows example measurement signals at an LDO regulator with andwithout clamping circuitry; and

FIG. 6 shows a flow chart of an example method for reducing the recoverytime of a regulator subject to an overvoltage situation.

DESCRIPTION

As outlined above, voltage regulators may exhibit relatively longrecovery times subject to an overvoltage condition. The present documentis directed at the technical problem of reducing the recovery times ofsuch regulators subject to an overvoltage condition.

A typical LDO regulator 100 is illustrated in FIG. 1a . The LDOregulator 100 comprises an output amplification stage 103 at the outputand a differential amplification stage or differential input stage 101(also referred to as error amplifier) at the input. A first input (fb)107 of the differential input stage 101 receives a fraction of theoutput voltage V_(out) determined by the voltage divider 104 comprisingresistors R0 and R1. The second input (ref) to the differential inputstage 101 is a stable voltage reference V_(ref) 108 (also referred to asthe bandgap reference or reference voltage). If the regulator outputvoltage V_(out) changes relative to the reference voltage V_(ref), thedrive voltage (also referred to as gate signal) to the pass transistorof the output amplification stage changes by a feedback mechanism calledthe main feedback loop to maintain a constant regulator output voltageV_(out).

The LDO regulator 100 of FIG. 1a further comprises an additionalintermediate amplification stage 102 configured to amplify thedifferential output voltage of the differential input stage 101. Assuch, an intermediate amplification stage 102 may be used to provide anadditional gain within the amplification path. Furthermore, theintermediate amplification stage 102 may provide a phase inversion.

In addition, the LDO regulator 100 may comprise an output capacitanceC_(out) (also referred to as output capacitor or stabilization capacitoror bypass capacitor) 105 parallel to the load 106. The output capacitor105 is used to stabilize the regulator output voltage V_(out) subject toa change of the load 106, in particular subject to a change of the loadcurrent I_(load).

FIG. 1b shows further details of an LDO regulator 100. The output stage103 typically comprises a pass transistor 111 which is configured toprovide the load current 116 at the regulator output voltage 115 to theload 106. The pass transistor 111 is coupled to the input voltage orsupply voltage 117. The pass transistor 111 is controlled via a controlsignal (e.g. a gate signal or gate voltage) 112, wherein the controlsignal is generated by an output driver 110.

The LDO regulator 100 of FIG. 1b further comprises a Miller capacitor113 which provides a feedback from the output (i.e. the drain) of thepass transistor 111 to the output of the differential input stage 101.Feedback is provided via the Miller capacitor 113 notably when changesoccur to the regulator output voltage 115, thereby affecting thedifferential output voltage 114 of the differential input stage 101.

FIG. 1c shows example measurement signals at the LDO regulator 100 ofFIG. 1b subject to an overvoltage condition. In particular, FIG. 1cshows an example curve of the load current 116, of the regulator outputvoltage 115 and of the differential output voltage 114. It can be seenthat subject to an increase of the load current, the regulator outputvoltage 115 may exhibit a relatively small dip below the target voltage135 (which depends on the reference voltage 108). Furthermore, it can beseen that subject to a reduction of the load current 116 (during thetime interval 121), the regulator output voltage 115 exhibits a peak.Such a situation may be referred to as an overvoltage condition.

As can be observed from FIG. 1c , the differential output voltage 114starts increasing after load removal. This is typically due to therelatively low bandwidth of the LDO regulator 100. It takes a certaintime interval 121 until the current through the pass transistor 111changes from IMAX to 0 and during this time interval 121, the outputcapacitor 105 is charged with the current through the pass transistor111. This leads to the increase of the regulator output voltage 115.Because the regulator output voltage 115 is above the target voltage135, the differential output voltage 114 is above zero during the timeintervals 121 and 122. This results in a decreased 2^(nd) stage outputvoltage (referred to as the intermediate voltage) and in an increased3^(rd) stage output (i.e. to an increase regulator output voltage 115)during the time interval 122. The outputs of the 2^(nd)/3^(rd) stage maydecrease/increase until the outputs are saturated at ground respectiveat the supply voltage 117, unless another excitation arrives. Theregulator output voltage 115 of the LDO regulator 100 decreases duringthe time interval 122 since the pass transistor 111 is completely offand since the load current (notably the current through the resistordivider 104) slowly discharges the output capacitor 105.

The differential output voltage 114 typically exhibits a relatively slowslewing due to a relatively low current capability of the differentialinput stage 101 and due to a relatively large Miller capacitor 113. Oncethe load 106 is totally removed, the only discharge path for the outputcapacitor 105 during the time intervals 122, 123 is via the resistordivider 104. At the end of the time interval 122, the regulator outputvoltage 115 reaches the target voltage 135, thereby causing thedifferential output voltage 114 to be discharged. This may be consideredto be the end of the overvoltage condition. However, due to therelatively low current capability of the differential input stage 101and due to a relatively large Miller capacitor 113, the discharge of thedifferential output voltage 114 may require a relatively long timeinterval 123. The 2^(nd) stage and 3^(rd) stage outputs are typicallystill clamped at minimum/maximum at the end of the time interval 122.

If there occurs another load request prior to the end of the timeinterval 123, all internal nodes of the LDO regulator 101 are typicallystill relatively far away from their operating points. In particular,the pass transistor 111 is completely off. As a result of this, a loadcurrent 116 which is requested from the LDO regulator 100 can only beprovided from the output capacitor 105, resulting in a relatively largedip of the regulator output voltage 115 until at time instant at whichthe pass transistor 111 goes back into its original operating point.

Such a situation may be avoided by preventing the differential outputvoltage 114 to substantially increase subject to an overvoltagecondition. An increase of the differential output voltage 114 may beprevented by clamping the output node of the differential input stage101 to a voltage which is only slightly higher (e.g. 5-10% or ˜10 mVhigher) than the closed loop regulated operating voltage of this outputnode.

FIG. 3 shows an example LDO regulator 100 with clamping circuitry 201,202, 203. The clamping circuitry 201, 202, 203 comprises clamp controlcircuitry 201, 203 which is configured to detect an overvoltagecondition of the output voltage 115 and which is configured to trigger aclamping unit 202 to clamp the differential output voltage 114 to acertain voltage level (referred to herein as the clamping voltage). Theclamping unit 202 is depicted in FIG. 2 as a clamping diode whichexhibits a diode voltage drop that corresponds to the clamping voltage.

FIG. 4 shows a circuit diagram of an example LDO regulator 100 withclamping circuitry 201, 202, 203. The second amplification stage 102 maycomprise a 2^(nd) stage upper transistor 321 and a 2^(nd) stage lowertransistor 322 which are arranged in series between the input voltage117 and ground. The 2^(nd) stage upper transistor 321 forms a currentmirror with a bias transistor 302, thereby copying a bias current from abias current source 301 to the 2^(nd) stage upper transistor 321(thereby providing the 2^(nd) stage comparator current, I2c 353). Thedifferential output voltage 114 of the differential input stage 101 isapplied to the gate of the 2^(nd) stage lower transistor 322. Anincrease of the differential output voltage 114 leads to an increase ofthe current through the 2^(nd) stage lower transistor 322, therebypulling the midpoint between the 2^(nd) stage upper transistor 321 andthe 2^(nd) stage lower transistor 322 down (once the current through the2^(nd) stage lower transistor 322 exceeds the 2^(nd) stage comparatorcurrent), i.e. thereby reducing the output voltage 354 of the 2^(nd)stage 102.

In a similar manner, the 3^(rd) stage 103 comprises a 3^(rd) stage uppertransistor 331 and a 3^(rd) stage lower transistor 332 which arearranged in series between the input voltage 117 and ground. The outputvoltage 354 of the 2^(nd) stage 102 is applied to the gate of the 3^(rd)stage lower transistor 332. A decrease of the output voltage 354 of the2^(nd) stage 102 leads to an increase of the gate signal 112 which isapplied to the gate of the pass transistor 111 (which comprises a p-typemetaloxcide semiconductor, MOS, transistor), thereby closing the passtransistor 111. The gate signal 112 may be biased using an auxiliarytransistor 303.

In FIG. 3, the clamping circuitry 201, 202, 203 comprises a sensing unit203 which is configured to sense an overvoltage condition. Inparticular, the sensing unit 203 is configured to detect a situation,when the pass transistor 111 is being turned off completely, due to anincrease of the gate signal 112. In the illustrated example, the sensingunit 203 comprises an upper sensing transistor 341 and a lower sensingtransistor 342 which are arranged in series between the input voltage117 and ground. The output voltage 354 of the 2^(nd) stage 102 isapplied to the gate of the lower sensing transistor 342 (as is the casefor the 3^(rd) stage lower transistor 332). Furthermore, the uppersensing transistor 341 forms a current mirror with the bias transistor302 as is the case for the auxiliary transistor 303 which is arranged inseries with the 3^(rd) stage lower transistor 332 between the inputvoltage 117 and ground). As such, a sensing comparator current, Icomp351 is provided at the upper sensing transistor 341, wherein the sensingcomparator current 351 is compared to a current through the lowersensing transistor 342, which varies in dependence of the output voltage354 of the 2^(nd) stage 102 (as is the case for the current through the3^(rd) stage lower transistor 332). Hence, the voltage level at themidpoint between the upper sensing transistor 341 and the lower sensingtransistor 342 (which is referred to as the sensing midpoint 355) may beused as an indication on whether the pass transistor 111 is being turnedoff or not.

The upper sensing transistor 341 and the auxiliary transistor 303 may bedesigned such that sensing comparator current 351 triggers clamping ofthe differential output voltage 114 prior to a time instant at which thepass transistor 111 is fully turned off. This may be achieved byselecting the upper sensing transistor 341 to have an increasedwidth-to-length ratio (e.g. by a factor of 2) compared to the auxiliarytransistor 303. The 3^(rd) stage lower transistor 332 and the lowersensing transistor 342 may be selected to have the same width-to-lengthratio.

The clamping circuitry 201, 202, 203 further comprises a triggering unit201 which is configured to trigger the clamping unit 202 to clamp thedifferential output voltage 114 to a clamping voltage, in dependence ofthe voltage level at the sensing midpoint 355. In the illustratedexample, the triggering unit 201 comprises the first and secondtransistors 343, 344 which trigger the clamping transistor 352 to be on,if the voltage level at the sensing midpoint 355 above a pre-determinedcurrent threshold. As a result of this, the differential output voltage114 is clamped to the gate-source voltage 352 of the clamping transistor345. Otherwise, the triggering unit 201 maintains the clampingtransistor 345 in off-state, such that the clamping circuitry 201, 202,203 has no impact on the differential output voltage 114.

In other words, the circuitry, which comprises the midpoint between theupper sensing transistor 341 and the lower sensing transistor 342 (i.e.the sensing midpoint 355), the lower sensing transistor 342, theclamping transistor 345, as well as the first and second transistors343, 344, acts like a brake mechanism, at time instants at which thedifferential output voltage 114 increases due to an imbalance at theinput of the differential pair of the differential input stage 101. As aresult of the brake mechanism, the differential output voltage 114 canonly increase until the current which is sensed by the lower sensingtransistor 342 reaches the current Icomp 351 through the upper sensingtransistor 341, wherein Icomp is set by the bias current source 301 andthe mirror ratio of the current mirror formed by the transistors 302,341.

Once the current through the lower sensing transistor 342 reaches Icomp351, the increase of the differential output voltage 114 is stopped. Dueto the relatively high gain of this feedback loop, the accuracy of theclamping is relatively high and the clamping voltage may be set to beclose to the operating point of normal operation.

In the illustrated example, the 3^(rd) stage 103 comprises of diodeconnected PMOS transistor 331 and the auxiliary transistor 303 whichacts as an additional current source and which helps for biasing the3^(rd) stage 103 under no load condition. Without the auxiliarytransistor 303, the 3^(rd) stage 103 would not exhibit any currentflowing under a no load condition. This might cause instability andaccuracy issues under a no load condition.

In case of an overvoltage event, the differential output voltage 114increases and the output voltage 354 of the 2^(nd) stage 102 decreases,thereby increasing the gate voltage 112 for the pass transistor 111, inorder to ensure that no current is injected to the output of the LDOregulator 100 through the pass transistor 111. In such a situation, theonly current through the 3^(rd) stage lower transistor 332 is due to thecurrent provided by the auxiliary transistor 303.

However, once the output voltage 354 of the 2^(nd) stage 102 falls belowa certain level, the current through the auxiliary transistor 303 fallsbelow its normal operation value. The sensing comparator current Icomp351 may be replica of the current through the auxiliary transistor 303.The upper sensing transistor 341 and the auxiliary transistor 303 may bedesigned such that if the current through the auxiliary transistor 303falls below a certain level, the gates of the first and secondtransistors 343, 344 are high, thereby switching on the clampingtransistor 344 and thereby clamping the differential output voltage 114to the V_(GS) 352 of clamping transistor 345. This clamping voltageconstitutes the steady state operating point during an overvoltagesituation.

As indicated above, the size of the lower sensing transistor 342 may bethe same as the size of the 3^(rd) stage lower transistor 332. The sizeof the upper sensing transistor 341 may be K, with K>1, times higherthan the size of the auxiliary transistor 303.

Under overvoltage condition, the current at the gate of the passtransistor 111 is close to zero. The current through the auxiliarytransistor 303 and through the 3^(rd) stage lower transistor 332 startsgoing low, and the current through this branch is sensed by the lowersensing transistor 342. Once the current through the lower sensingtransistor 342 starts getting less than the current through the 3^(rd)stage lower transistor 332, clamping may be activated. Due to a loopfrom the output of the differential input stage 101 to the sensingmidpoint 355 (via the intermediate stage 102 and/or via the driver stage203), and back to the output of the differential input stage 101 (viathe first and second transistor 343, 344), the current through the3^(rd) stage lower transistor 332 stops at the current through the uppersensing transistor 341 by not letting the differential output voltage114 increase further. The gain of this loop influences (e.g. determines)the clamping voltage. In particular, the loop determines or sets thegate-source voltage 352 of the clamping transistor 345, thereby settingthe voltage to which the output of the differential input stage 101 isclamped.

FIG. 4 shows a circuit diagram of an example LDO regulator 100 with adifferent type of clamping circuitry 201, 202, 203. In particular, adifferent type of sensing unit 203 is used for generating theovervoltage indication at a sensing midpoint 355. The sensing circuit203 makes use of diodes 441, wherein the diode voltage drop maycorrespond to the threshold voltage of the pass transistor 111. Thesensing unit 203 further comprises a comparator transistor 444 which isarranged in series with a current source 442 between the gate voltage112 of the pass transistor 111 and ground. The current source 442provides a reference or comparator current Icomp 351. The gate of thecomparator transistor 444 is coupled to the midpoint between the twodiodes 441 and a further current source 443.

Subject to an overvoltage situation, the gate voltage 112 increases,thereby increasing the current through the comparator transistor 444.Once this current is above Icomp 351, the sensing midpoint 355 betweenthe comparator transistor 444 and the current source 442 goes high, andclamping is triggered. In other words, during an overvoltage event, thegate voltage 112 is increased through the regulation loop of theregulator 100 to ensure that the pass transistor 111 is off and that theoutput of the regulator 100 can be discharged by the load 106. Clampcontrol output, i.e. the gates of the first and second transistors 344,343, is high only if the gate voltage 112 is high enough so that acurrent flowing through the comparator transistor 444 is higher thanIcomp 351. In normal operation, when the gate voltage 112 is relativelylow no current is flowing through clamp control, thereby keeping thegates of the first and second transistors 344, 343 low.

FIG. 5 shows example experimental results with and without clamping. Theload current exhibits an increase to IMAX at time instant 501, adecrease to 0A at time instant 502 and a re-increase to IMAX at timeinstant 503. In case of clamping the differential output voltage 114 isclamped (see curve 521), whereas the differential output voltage 114increases substantially if no clamping is used (see curve 522). As aresult of clamping a dip of the regulator output voltage 115 can bereduced (curve 511) compared to a case without clamping (curve 512). Inother words, if the differential output voltage 114 is clamped at avalue close to the normal operating point using the clamping circuitry201, 202, 203, an output load request after an overvoltage event resultsin dips at the regulator output voltage 114 which are relatively small(comparable with normal operation transient load responses).

FIG. 6 shows a flow chart of an example method 600 for providing a loadcurrent 116 at a regulator output voltage 115 to a load 106. The method600 comprises providing 601 a differential output voltage 114 based on adifference between a reference voltage 108 and a feedback voltage 107derived from the regulator output voltage 115. Typically the feedbackvoltage 107 is proportional to the regulator output voltage 115.Furthermore, the method 600 comprises generating 602 a gate signal 112based on the differential output voltage 114 (typically using an outputdriver 110). In addition, the method 600 comprises providing 603 theload current 116 in dependence of the gate signal 112 using a passtransistor 111.

The method 600 further comprises sensing 604 an overvoltage indicationwhich indicates that the pass transistor 111 is being turned off.Furthermore, the method 600 comprises clamping 605 the differentialoutput voltage 114 to a clamping voltage 352, if the overvoltageindication indicates that the pass transistor 111 is being turned off.

It should be noted that the description and drawings merely illustratethe principles of the proposed methods and systems. Those skilled in theart will be able to implement various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and embodiment outlined in the present document are principallyintended expressly to be only for explanatory purposes to help thereader in understanding the principles of the proposed methods andsystems. Furthermore, all statements herein providing principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass equivalents thereof.

What is claimed is: 1) A regulator for providing a load current at aregulator output voltage to a load at an output of the regulator,wherein the regulator comprises a differential input stage configured toprovide a differential output voltage based on a reference voltage andbased on the regulator output voltage; an output driver configured togenerate a control signal based on the differential output voltage; apass transistor configured to provide the load current in dependence ofthe control signal; and clamping circuitry configured to sense anovervoltage indication which indicates that the pass transistor is beingturned off; and clamp the differential output voltage to a clampingvoltage, if the overvoltage indication indicates that the passtransistor is being turned off. 2) The regulator of claim 1, wherein theclamping circuitry is configured to sense the overvoltage indication bydetermining a mirrored version of the control signal. 3) The regulatorof claim 1, wherein the pass transistor is coupled to an input voltage;the output driver comprises an auxiliary transistor and a lower drivertransistor which are arranged in series between the input voltage andground; the control signal is provided at a midpoint between theauxiliary transistor and the lower driver transistor, referred to as acontrol midpoint; and a voltage applied to a gate of the lower drivertransistor depends on the differential output voltage. 4) The regulatorof claim 3, wherein the output driver further comprises an upper drivertransistor; a source of the upper driver transistor is coupled to theinput voltage; and a drain and a gate of the upper driver transistor arecoupled to the control midpoint. 5) The regulator of claim 3, whereinthe regulator comprises a bias current source configured to provide abias current; the regulator comprises a bias transistor which isarranged in series with the bias current source between the inputvoltage and ground; and the auxiliary transistor and the bias transistorform a current mirror. 6) The regulator of claim 5, wherein the clampingcircuitry comprises an upper sensing transistor and a lower sensingtransistor which are arranged in series between the input voltage andground; gates of the lower driver transistor and the lower sensingtransistor are coupled to one another; gates of the auxiliary transistorand the upper sensing transistor are coupled to one another; and theovervoltage indication is provided at a midpoint between the uppersensing transistor and the lower sensing transistor. 7) The regulator ofclaim 6, wherein a size of the upper sensing transistor is greater thana size of the auxiliary transistor; and a size of the lower sensingtransistor is equal to the size of the lower driver transistor. 8) Theregulator of claim 3, wherein the clamping circuitry comprises acomparator transistor and a reference current source which is configuredto provide a reference current; the comparator transistor and thereference current source are arranged in series between the controlmidpoint and ground; a gate of the comparator transistor is coupled toan offset version of the input voltage; and the overvoltage indicationis provided at a midpoint between the comparator transistor and thereference current source. 9) The regulator of claim 8, wherein theoffset version of the input voltage is generated using one of morediodes which are arranged in a forward biased manner between the inputvoltage and the gate of the comparator transistor. 10) The regulator ofclaim 1, wherein the clamping circuitry comprises a clamping diode whichis set to couple an output of the differential input stage to ground, ifthe overvoltage indication indicates that the pass transistor is beingturned off; and the clamping voltage depends on a diode voltage drop atthe clamping diode. 11) The regulator of claim 10, wherein theovervoltage indication takes on a low level and a high level; theclamping diode comprises a clamping transistor; a gate of the clampingtransistor is coupled to the output of the differential input stage; asource of the clamping transistor is coupled to ground; the clampingcircuitry is configured to couple or decouple a drain of the clampingtransistor to or from the gate of the clamping transistor in dependenceof the level of the overvoltage indication. 12) The regulator of claim11, wherein the clamping circuitry comprises a first transistor and asecond transistor; a drain of the first transistor is coupled to a nodeat which the overvoltage indication is provided; a drain of the secondtransistor is coupled to the output of the differential input stage;gates of the first transistor and the second transistor are coupled toone another; the gate of the first transistor is coupled to the drain ofthe first transistor; a source of the first transistor is coupled toground; and a source of the second transistor is coupled to the drain ofthe clamping transistor. 13) The regulator of claim 1, wherein theregulator comprises an intermediate amplification stage which is coupledto an output of the differential input stage and which is configured togenerate an intermediate voltage based on the differential outputvoltage; and the output driver is configured to generate the controlsignal based on the intermediate voltage. 14) The regulator of claim 1,wherein the differential input stage is configured to provide thedifferential output voltage based on a difference between the referencevoltage and a feedback voltage derived from the regulator outputvoltage. 15) A method for providing a load current at a regulator outputvoltage to a load, wherein the method comprises the steps of: —providinga differential output voltage based on a difference between a referencevoltage and a feedback voltage derived from the regulator outputvoltage; generating a control signal based on the differential outputvoltage; providing the load current in dependence of the control signalusing a pass transistor; sensing an overvoltage indication whichindicates that the pass transistor is being turned off; and clamping thedifferential output voltage to a clamping voltage, if the overvoltageindication indicates that the pass transistor is being turned off. 16) Amethod for providing a regulator for providing a load current at aregulator output voltage to a load at an output of the regulator,wherein the regulator comprises the steps of: providing a differentialinput stage to provide a differential output voltage based on areference voltage and based on the regulator output voltage; providingan output driver to generate a control signal based on the differentialoutput voltage; providing a pass transistor to provide the load currentin dependence of the control signal; and providing clamping circuitry tosense an overvoltage indication which indicates that the pass transistoris being turned off; and clamp the differential output voltage to aclamping voltage, if the overvoltage indication indicates that the passtransistor is being turned off. 17) The method of claim 16, furthercomprising the step of: sensing by the clamping circuitry theovervoltage by determining a mirrored version of the control signal. 18)The method of claim 16, wherein the pass transistor is coupled to aninput voltage; the output driver comprises an auxiliary transistor and alower driver transistor which are arranged in series between the inputvoltage and ground; the control signal is provided at a midpoint betweenthe auxiliary transistor and the lower driver transistor, referred to asa control midpoint; and a voltage applied to a gate of the lower drivertransistor depends on the differential output voltage. 19) The method ofclaim 18, wherein the output driver further comprises an upper drivertransistor; a source of the upper driver transistor is coupled to theinput voltage; and a drain and a gate of the upper driver transistor arecoupled to the control midpoint. 20) The method of claim 18, wherein theregulator comprises a bias current source to provide a bias current; theregulator comprises a bias transistor which is arranged in series withthe bias current source between the input voltage and ground; and theauxiliary transistor and the bias transistor form a current mirror. 21)The method of claim 20, wherein the clamping circuitry comprises anupper sensing transistor and a lower sensing transistor which arearranged in series between the input voltage and ground; gates of thelower driver transistor and the lower sensing transistor are coupled toone another; gates of the auxiliary transistor and the upper sensingtransistor are coupled to one another; and the overvoltage indication isprovided at a midpoint between the upper sensing transistor and thelower sensing transistor. 22) The method of claim 21, wherein a size ofthe upper sensing transistor is greater than a size of the auxiliarytransistor; and a size of the lower sensing transistor is equal to thesize of the lower driver transistor. 23) The method of claim 18, whereinthe clamping circuitry comprises a comparator transistor and a referencecurrent source to provide a reference current; the comparator transistorand the reference current source are arranged in series between thecontrol midpoint and ground; a gate of the comparator transistor iscoupled to an offset version of the input voltage; and the overvoltageindication is provided at a midpoint between the comparator transistorand the reference current source. 24) The method of claim 23, whereinthe offset version of the input voltage is generated using one of morediodes which are arranged in a forward biased manner between the inputvoltage and the gate of the comparator transistor. 25) The method ofclaim 16, wherein the clamping circuitry comprises a clamping diodewhich is set to couple an output of the differential input stage toground, if the overvoltage indication indicates that the pass transistoris being turned off; and the clamping voltage depends on a diode voltagedrop at the clamping diode. 26) The method of claim 25, wherein theovervoltage indication takes on a low level and a high level; theclamping diode comprises a clamping transistor; a gate of the clampingtransistor is coupled to the output of the differential input stage; asource of the clamping transistor is coupled to ground; the clampingcircuitry to couple or decouple a drain of the clamping transistor to orfrom the gate of the clamping transistor in dependence of the level ofthe overvoltage indication. 27) The method of claim 26, wherein theclamping circuitry comprises a first transistor and a second transistor;a drain of the first transistor is coupled to a node at which theovervoltage indication is provided; a drain of the second transistor iscoupled to the output of the differential input stage; gates of thefirst transistor and the second transistor are coupled to one another;the gate of the first transistor is coupled to the drain of the firsttransistor; a source of the first transistor is coupled to ground; and asource of the second transistor is coupled to the drain of the clampingtransistor. 28) The method of claim 16, wherein the regulator comprisesan intermediate amplification stage which is coupled to an output of thedifferential input stage and to generate an intermediate voltage basedon the differential output voltage; and the output driver generates thecontrol signal based on the intermediate voltage. 29) The method ofclaim 16, wherein the differential input stage provides the differentialoutput voltage based on a difference between the reference voltage and afeedback voltage derived from the regulator output voltage.